Lead and lead-alloy (positive) electrodes, are used extensively in the electrowinning of copper, zinc, manganese, nickel and other metals from sulfuric acid solutions. The use of lead and lead-alloys in such applications is based upon their general ability to withstand prolonged exposure to sulfuric acid under highly oxidizing conditions. Lead and lead-alloy electrodes, usually in the form of cast plates as described in U.S. Pat. No. 4,124,482, and typically containing alloying constituents such as Ag, Ca, Sn and Sb, are expected to endure periods of up to 4 years under such harsh acidic conditions. The degradation of these electrodes is primarily due to intergranular corrosion, which occurs as a result of local volumetric changes associated with lead-sulfuric to lead-oxide transitions at the intersection of internal grain boundaries with the free surface of the electrodes. This results in a local compromise of the protective lead-oxide film, and subsequent propagation of corrosive attack into the grain boundaries, and ultimately, general loss of electrode metal via spalling and grain dropping. Such loss of electrode material, in addition to compromising the structural integrity of the electrode, results in contamination of the electrolyte by lead and other electrode alloying constituents, which ultimately limits the purity of the metal deposit which can be achieved during the electrowinning process.
Numerous studies have shown that certain `special` grain boundaries, described on the basis of the well-established `Coincidence Site Lattice` model of interface structure (Kronberg and Wilson, 1949.sup.1 as lying within .gamma..theta. of .SIGMA. where .SIGMA..English Pound.29 and .gamma..theta..English Pound.15.SIGMA..sup.-1/2 (Brandon, 1966).sup.1 are highly resistant to intergranular degradation processes such as corrosion and cracking. In a previous U.S. patent (Palumbo, 1997).sup.3, a thermomechanical process is disclosed for increasing the population of such special grain boundaries in commercial austenitic Fe and Ni-based stainless alloys from approximately 20%-30% to levels in excess of 60%; such an increase resulting in significantly improved resistance to intergranular degradation processes such as intergranular corrosion and stress corrosion cracking. In more recent patent applications (Palumbo, Lehockey, and Brennenstuhl).sup.4, thermomechanical processes are disclosed for achieving such improvements with lead alloys commonly used as electrodes in conventional lead-acid batteries. The patents, applications and publications discussed above and identified by footnotes are incorporated by reference herein, for their disclosures on alloy interfacial structure.